1. Field of the Invention
This invention relates to an optical system for scanning with a laser beam in a radiation image read-out system wherein a stimulable phosphor sheet carrying a radiation image stored therein is scanned with a laser beam emitted from a laser beam source to read out the radiation image stored in the stimulable phosphor sheet.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or simply as a phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
This radiation image recording and reproducing system using the stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing, particularly for diagnostic purposes. In more detail, since the amount of light emitted upon stimulation after the radiation energy is stored in the phosphor varies over a very wide range in proportion to the amount of energy stored therein, it is possible to obtain an image having desired density regardless of the amount of exposure of the phosphor to the radiation by reading out the emitted light with an appropriate read-out gain and converting it to an electric signal to reproduce a visible image on a recording medium or a display device. The electric signal may further be processed as desired to obtain a radiation image suitable for viewing, particularly for diagnostic purposes. This is very advantageous in practical use.
In the aforesaid radiation image recording and reproducing system, the read-out step for reading out the radiation image from the stimulable phosphor sheet carrying the radiation image stored therein is conducted by exposing the stimulable phosphor sheet to stimulating rays which cause it to emit light in proportion to the stored energy of the radiation, and photoelectrically detecting the emitted light. Typically, the read-out step is conducted by two-dimensionally scanning the stimulable phosphor sheet with a light beam such as a laser beam which causes it to emit light in proportion to the stored energy of the radiation, and sequentially detecting the emitted light and converting it into an image signal by use of a photodetector such as a photomultiplier. In this case, the two-dimensional scanning is carried out by scanning the stimulable phosphor sheet with the light beam in a main scanning direction and a sub-scanning direction. For this purpose, use is made of a scanning optical system in which the stimulable phosphor sheet is moved in one direction and, at the same time, scanned with the light beam in the direction normal to the moving direction of the stimulable phosphor sheet, or a scanning optical system wherein the stimulable phosphor sheet is maintained stationary and scanned with the light beam in both the main scanning direction and the sub-scanning direction.
In the above-described radiation image read-out system, a gas laser tube (including a gas ion laser tube) emitting an Ar.sup.+ laser beam, a Kr.sup.+ laser beam, an He-Ne laser beam, or the like is normally used as the laser beam source. In a system using the gas laser tube as a light source, spherical mirrors are used as a resonator for convenience of operation, i.e. for facilitating oscillation. When the spherical mirrors are positioned in confocal relation to each other (i.e. when the spherical mirrors are positioned face to face so that the focal points thereof coincide with each other), the divergence angle .theta..sub.0 (radian) of the gas laser beam is approximately represented by ##EQU1## wherein ##EQU2## d denotes the distance between the mirrors, and .lambda. designates the wavelength of the laser beam. In general, the divergence angle .theta..sub.0 of the gas laser beam is very much smaller than the divergence angle of a semiconductor laser beam. On the other hand, in order to improve the final image forming effect, it is necessary that the laser beam impinging upon an image forming lens be diverged. In the system using the gas laser tube as the light source, since the divergence angle .theta..sub.0 of the laser beam is small as described above, a beam expander is inserted between the gas laser tube and the image forming lens to expand the beam diameter of the laser beam. The beam expander normally comprises a combination of two convex lenses, and the beam diameter of the laser beam incident on the beam expander is expanded by the beam expander to an extent depending on the ratio between the focal lengths of the convex lenses.
However, in the radiation image read-out system as described above, since the surface of the stimulable phosphor sheet is formed by a smooth surface exhibiting a reflectivity of about 4%, the laser beam emitted from the gas laser tube to the stimulable phosphor sheet is reflected by the surface of the stimulable phosphor sheet, and a part of the reflected laser beam returns back to the gas laser tube. As a result, oscillation of the gas laser tube becomes inconsistent, and the laser beam output is disturbed. Therefore, the image quality of the finally read-out image is adversely affected. Further, in the aforesaid system, since the beam expander is used, the laser beam reflected from the stimulable phosphor sheet is again reflected by the lenses of the beam expander to the stimulable phosphor sheet. This also adversely affects the image quality of the finally read-out image.
FIG. 1 schematically shows the above-mentioned condition in the conventional radiation image read-out system. A laser beam emitted from a gas laser tube 1 is expanded by a beam expander 2 and then converged by an image forming lens (not shown). The converged laser beam is directed by a scanning optical member 3 such as a galvanometer mirror via a reflection mirror 4 onto the surface of a stimulable phosphor sheet 5. When the laser beam is directed by the scanning optical member 3 along the optical path A or C onto an end portion of a scan line on the stimulable phosphor sheet 5, the angle .phi.' or .phi." of the incident light with respect to the surface of the stimulable phosphor sheet 5 in a plane 7 in which the incident light beams along the optical paths A and C lie is different from and far apart from 90.degree.. In this case, therefore, the laser beam reflected from the surface of the stimulable phosphor sheet 5 does not go along the same optical path as that of the incident light. On the other hand, when the laser beam is directed by the scanning optical member 3 along the optical path B onto the middle point of a scan line on the stimulable phosphor sheet 5, the angle .phi. of the incident light with respect to the surface of the stimulable phosphor sheet 5 in the plane 7 is equal to 90.degree.. In this case, when the angle .theta. of the incident light with respect to the surface of the stimulable phosphor sheet 5 in the direction 8 normal to the scanning direction indicated by the arrow 6 also becomes equal to 90.degree., the light reflected from the surface of the stimulable phosphor sheet 5 advances along the same optical path as that of the incident light. A part of the reflected light returning along the optical path B is reflected by a convex lens 9 positioned on the stimulable phosphor sheet side of the beam expander 2. Thus the reflected light again impinges upon the stimulable phosphor sheet 5, and causes a disturbance in the finally read-out image. Further, a part of the reflected light returning from the stimulable phosphor sheet 5 along the optical path B passes through the beam expander 2 to the gas laser tube 1, and renders the oscillation of the gas laser tube 1 inconsistent. Accordingly, the laser beam output of the gas laser tube 1 becomes inconsistent, and the image quality of the finally read-out image is adversely affected. FIG. 2 is a side view showing the reflection mirror 4 and the stimulable phosphor sheet 5 in FIG. 1, as viewed in the scanning direction indicated by the arrow D.
The adverse effects of the reflected light as described above present a very real problem in the aforesaid radiation image recording and reproducing system which is required to reproduce a radiation image having a high image quality, particularly a high diagnostic efficiency and accuracy.
In order to solve the above-mentioned problem, it has heretofore been known, for example, to insert a polarizing plate and a quarter wave plate between the beam expander and the scanning mirror. In this method, a linearly polarized laser beam emitted from a gas laser tube is passed through the polarizer the polarization axis of which is aligned with that of the linearly polarized laser beam, and then passed through the quarter wave plate to yield elliptically polarized light. When the elliptically polarized light is reflected by the surface of a stimulable phosphor sheet and returned along the same optical path as that of the elliptically polarized light incident on the stimulable phosphor sheet, the elliptically polarized light thus reflected is passed again through the quarter wave plate and converted thereby to linear polarized light. However, since the plane of polarization of the reflected linear polarized light is rotated by 90.degree. with respect to the plane of polarization of the original incident linear polarized light, the reflected linear polarized light is shielded by the polarizer and is not fed back to the beam expander and the gas laser tube.
The method just described is nevertheless disadvantageous since, because of the insertion of the optical members such as the polarizer and the quarter wave plate in the optical system, it takes much time for optical adjustment to be carried out, and the cost of the system becomes high.